The Role of Surface Finish in Improving Part Performance

Mass media finishing techniques can be used to improve part performance and service life

Jack Clark Owner/Consultant, Surface Analytics

David A. DavidsonDeburring/Surface Finishing Specialist

Mass finishing processes have been widely adopted throughout industry as the optimum methodology for producing controlled edge and surface finish effects on many types of machined and fabricated components. American industry has long been on the forefront of aggressively deploying these methods to improve edge and surface finishing operations.

All too often, situations still exist where archaic and even primitive hand or manual finishing methods are used to produce edge and surface finishing improvements. This is not to say that some industrial part applications are not going to require a manual deburring operation—some do. In many cases, however, hand or manual methods are still being used because more automated or mechanized methods have not been considered or adequately investigated.

An often-observed dichotomy in precision manufacturing operations is that many manufacturers, after spending vast sums on CNC machining equipment to produce parts to very precise tolerances and specifications consistently,

in the end hand off these expensive parts to a deburring and finishing department that uses hand methods, with all the inconsistency, non-uniformity, rework and worker-injury potential that implies.

Even when manual methods can’t be completely eliminated, mass-media finish techniques can and should be used to produce an edge and surface finish uniformity that simply cannot be duplicated with manual or single-point-of-contact methods. Developing an overall edge and surface finish continuity and equilibrium can have a significant effect on performance and service life of critical components.

In recent years, mass-media finishing processes have gained widespread acceptance in many industries, primarily as a technology for reducing the costs of producing edge and surface finishes. The economics are especially striking when manual deburring and finishing procedures are minimized or eliminated.

The first casualty of overreliance on a manual deburring and finishing approach is the investment the manufacturer has made, often in the millions, for precise and computer-controlled manufacturing equipment. The idea behind this investment was to have the ability to produce parts that are uniformly and carefully manufactured to exacting specifications and tolerances. At this point, in too many cases, the parts are then sent to manual deburring and finishing procedures that will all but guarantee that no two parts will ever be alike.

Moreover, the increased complexity and precise requirements of mechanical products have reinforced the need for accurately producing and controlling the surface finish of manufactured parts. Variations in the surface texture can influence a variety of performance characteristics. The surface finish can affect the ability of the part to resist wear and fatigue; to assist or destroy effective lubrication; to increase or decrease friction and/or abrasion with mating parts; and to resist corrosion. As these characteristics become critical under certain operating conditions, the surface finish can dictate the performance, integrity and service life of the component.

To understand how edge condition and surface topography improvement can impact part performance, some understanding of how part surfaces developed from common machining, grinding, honing, and other methods can negatively influence part function over time. A number of factors are involved:The role of mass-finishing processes (barrel, vibratory, centrifugal and spindle finishing) as a method for removal of burrs, developing edge contour and smoothing and polishing parts has been well estab

To understand how edge condition and surface topography improvement can impact part performance, some understanding of how part surfaces developed from common machining, grinding, honing, and other methods can negatively influence part function over time. A number of factors are involved:lished and documented for many years. Less well known and less clearly understood is the role specialized variants of these types of processes can play in extending the service life and performance of critical components or tools in demanding manufacturing or operational applications.To understand how edge condition and surface topography improvement can impact part performance, some understanding of how part surfaces developed from common machining, grinding, honing, and other methods can negatively influence part function over time. A number of factors are involved:

1) Positive vs. Negative Surface Skewness: The skew of surface profile symmetry can be an important surface attribute. Surfaces are typically characterized as being either negatively or positively skewed. This surface characteristic is referred to as Rsk (Rsk—skewness—the measure of surface symmetry about the mean line of a profilometer trace). Conventionally machined parts usually display a concentration of surface peaks above this mean line, a positive skew (Fig 1).

Thus it is axiomatic that almost all surfaces produced by common machining and fabrication methods are positively skewed. These positively skewed surfaces have an undesirable effect on the bearing load of surfaces, negatively impacting the performance of parts involved in applications where there is substantial surface-to-surface contact. Specialized high-energy finishing procedures can truncate these surface profile peaks and achieve negatively skewed surfaces (Fig 2) that are plateaued, presenting a much higher surface bearing contact area.

Application: The transition from Gaussian (Fig. 1) honed surfaces to carefully specified plateaued surfaces (Fig 2.) in diesel fuel injector bore and mating timing plungers resulted in eliminating a multi-million dollar warrantee problem for an over-the-road diesel engine manufacturer. One in six injectors would "stick" and usually allow raw fuel to flow, misfire, and sometimes cause that cylinder to seize. The plateaued surface has a high "bearing ratio" to distribute the high fuel pressure loads and a uniform valley lay that effectively distributes fuel (the fuel oil is the system’s lubricant). Today, these are "standard" two or three proces-honing techniques and mass finishing processes used by all high-performance fuel injector manufacturers that eliminate failures due to part-to-part contact through the lubricant film.

2) Directional vs. Random (Isotropic) Surface Texture Patterns: Somewhat related to surface texture skewness in importance is the directional nature of surface textures developed by typical machining and grinding methods. These machined surfaces are characterized by tool marks or grinding patterns that are aligned and directional in nature (Fig. 3).

It has been established that tool or part life and performance can be substantially enhanced if these types of surface textures can be altered into one that is more random in nature. Post-machining processes that utilize free or loose abrasive materials in a high-energy context can alter the machined surface texture substantially, not only reducing surface peaks, but generating a surface in which the positioning of the peaks has been altered appreciably. These "isotropic" surface effects (Fig 4.) have been demonstrated to improve part wear and fracture resistance, bearing ratio and improve fatigue resistance.

Application: Figures 3 and 4 are automotive camshaft rollers that connect the rotating action of the cam to the reciprocating valve. The loads on this surface are high and must be transmitted through a consistent layer of oil to maintain a rolling action between the moving elements. The directionally ground roller (Fig 3.) was contacting the cam lobe and failing both the roller and the camshaft to result in an engine manufacture liability in the many millions of dollars.

Each failure would force the manufacturer to, in the field, replace the camshaft, rollers and, many times, other normally non-related components because of wear debris moved throughout the engine via the lubrication system. The random, high load bearing surface shown in Fig 4. maintains a predictable oil film and keeps the rollers from contacting the cam and, therefore, resisting wear.

3) Residual Tensile Stress vs. Residual Compressive Stress: Many machining and grinding processes tend to develop residual tensile stresses in the surface area of parts. These residual tensile stresses make parts susceptible to premature fracture and failure when repeatedly stressed. High-energy mass finishing processes can be implemented to modify this surface stress condition and replace it with uniform residual compressive stresses.

Many manufacturers have discovered that as mass finishing processes have been adopted, put into service, and the parts involved have developed a working track record, an unanticipated development has taken place—their parts are better. In the case of automotive valve springs (Fig 5.): they last longer in service, are less prone to metal fatigue failure and, from a quality assurance perspective, are much more predictably consistent and uniform.

Application: Springs of any type have a predictable life based on the material, shape, movement range, load and interference/resonance with a mating spring in multiple spring applications. High-performance automotive valve springs are some of the most stressed movement control applications. The spring is controlling the vertical movement of a mass (the valve) over a distance (over 0.5" or 12.7 mm) at very high frequencies (5000–10,000 openings a minute). The valve spring is typically a drawn wire wound into the coil shape and "shot peened" for "stress relief".

The peening process consists of steel balls pounded into the spring’s surface during an aggressive tumbling or wheel blast action. This does impart some compressive stress but in a macro form. Even with this level of compressive residual stress the spring will fracture unpredictably in many high-performance applications requiring the users (race teams) to change the springs for at least every event during a racing season. The same spring subjected to a high-energy mass finishing process will result in the spring lasting easily 5–10 times longer before eventual failure. Even more important is that the pressure the spring exerts on the system is consistent, allowing for predictable engine performance over the entire life of the spring. The fact that the spring will last this extended time compared to a "standard" spring results in reduced cylinder head maintenance, no premature failures that catastrophically can ruin the engine, and higher frequency operation (higher RPM equals more horsepower).

4) Mixed Bag—Compatible Surfaces but Different Function: The "draw and iron" process used to make aluminum beverage cans is complicated, uses very fast production rates and requires tight punch and die tolerances. The punch drives a sheet of aluminum through a progressively smaller and smaller pack of dies to thin the material and form the can in one (1) stroke of the punch at the rate of 400 cans per minute. If the surface finish of the punch does not retain lubricant, the newly formed can cannot be "stripped" off the punch, damaging the can. If the finish on the dies is not such that they produce a bright can and do not allow aluminum "pickup," the cans OD will be unacceptable.

Also, due to surface specification, the tolerance between the punch and dies can be reduced and maintained, the wall thickness of the can be controlled and reduced. All these functions are dependent on the directionality and control of the surface finish on both components.

The punch must have the random isotropic surface discussed in #2 above and the dies must have a negatively skewed surface, discussed in #1 above, that is in the direction of the drawn material (perpendicular to the diameter of the die). When all these criteria are met, one can expect bright OD finished cans that strip easily (reducing scrap) and have a minimally thick wall. Wall thickness is not a trivial matter—savings of up to $10M per year per 0.001" (0.0254-mm) wall thickness reduction in a given can plant have been reported.

To summarize: Mass media finishing techniques (barrel, vibratory, centrifugal and spindle finish) can be used to improve part performance and service life, and these processes can be tailored or modified to amplify this effect. Although the ability of these processes to drive down deburring and surface finishing costs when compared to manual procedures is well known and documented, their ability to dramatically affect part performance and service life is not widely recognized, nor well understood.

Abundant opportunities exist for part performance and part life improvement with substantial economic advantages. All that is required is a more thoughtful and purposeful approach to the selection and implementation of the mass finishing processes available to manufacturers.

This article was first published in the November 2012 edition of Manufacturing Engineering magazine. Click here for PDF.